It is well known that densitometric sensing devices have been used in connection with printing presses to measure the printed density of the ink on a zone-by-zone basis along a test strip usually printed along one edge of the sheet. Standards are set for the desired density in each zone and for each color, and the measured densities are compared against the standards. If one or more of the measured density values are outside the desired range set by the standards, adjustments are made to the press, typically to the ink supply device supplying the particular color ink to the zone in question, until the measured density is brought into the desired range. Sheets are scanned periodically to assure that the densities remain as desired and thus the printed product will be of high quality.
One such scanning device is shown in Ott EP-OS 149 424. As disclosed in Ott, an information code is printed adjacent the test strip for accurate location of the appropriate measurement position. This feature is expensive and has the disadvantage that even more space is required on the printed sheet for densitometric sensing, a consideration which further reduces its desirability.
Another approach for scanning an ink test strip is described in published German patent application DE 36 31 204 and U.S. Pat. application Ser. No. 096,596. The scanning head disclosed in those documents includes a linear array of sensors disposed generally transverse to the test strip and having a span which is wider than the test strip such that some of the sensors will be over the test strip even if there is misalignment between the test strip and scanning direction of the head. Control means responsive to the signals from the individual sensors determines which of the sensors are over the test strip and utilizes the readings from those sensors as a measure of the density of the zone of the test strip being scanned. While such an approach provides reliable results, it requires more than one sensor in the test head, requires the sensors to be fairly accurately matched, and requires additional complexity in the control circuitry for determining which of the sensors are over the test strip.
Yet a further approach to postioning a densitometric sensing head with respect to an ink test strip is to provide the sensing head with a zig zag motion which continually traverses the strip. The zig zag pattern assures that the scanning head crosses the strip numerous times during each traverse. However, such an apparatus does not operate properly when the test strip is intermittent, i.e., when some of the patches within the test strip are unprinted. That is typically the case when a multicolored test strip is used but one or more of the colors in the multicolor strip is not printed in particular zones on a page. In that case, the test strip for that color in those zones will be unprinted, and the zig zag motion of the scanning head will serve to confuse the control circuitry because no test strip will be detected. In addition, some scanning heads are arranged to bear, by means of a wheel or runner, on the sheet being scanned. In that case, the zig zag motion of the scanning head tends to smear the fresh ink on the sheet as well as foul the wheel or runner of the scanning head with ink picked up from the strip.
In summary, with respect to these approaches, it is seen that one approach requires additional complexity of the test strip in order the guide the sensor head over the test strip, another requires additional complexity in the scanning head coupled with means for selecting the signals from the multiple sensors, and the third lacks adequate reliability in many cases.